Question: Simplify the following expression: $y = \dfrac{-5x^2- 21x+20}{-5x + 4}$
Explanation: First use factoring by grouping to factor the expression in the numerator. This expression is in the form ${A}x^2 + {B}x + {C}$ First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(-5)}{(20)} &=& -100 \\ {a} + {b} &=& &=& {-21} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $-100$ and add them together. Remember, since $-100$ is negative, one of the factors must be negative. The factors that add up to ${-21}$ will be your ${a}$ and ${b}$ When ${a}$ is ${4}$ and ${b}$ is ${-25}$ $ \begin{eqnarray} {ab} &=& ({4})({-25}) &=& -100 \\ {a} + {b} &=& {4} + {-25} &=& -21 \end{eqnarray} $ Next, rewrite the expression as $({A}x^2 + {a}x) + ({b}x + {C})$ $ ({-5}x^2 +{4}x) + ({-25}x +{20}) $ Factor out the common factors: $ x(-5x + 4) + 5(-5x + 4)$ Now factor out $(-5x + 4)$ $ (-5x + 4)(x + 5)$ The original expression can therefore be written: $ \dfrac{(-5x + 4)(x + 5)}{-5x + 4}$ We are dividing by $-5x + 4$ , so $-5x + 4 \neq 0$ Therefore, $x \neq \frac{4}{5}$ This leaves us with $x + 5; x \neq \frac{4}{5}$.